The present invention relates to a bond pad array for a complex IC, and more particularly, to a bond pad array that has a small pad pitch and good electro-migration.
Integrated circuit (IC) chips utilize metal pads for receiving and supplying signals to and from other circuitry. The pad is formed by a plurality of metal layers, usually in a rectangular shape, wherein some of the metal layers are utilized for carrying signals between the pads and other circuitry of the IC chip. Conductive lines formed from one or more of the metal layers connect the pads to the circuitry.
Pad sizes are relatively large compared to the chip, so the arrangement of pads limits the remaining space for circuitry. Pads are usually arranged around the edge of the IC chip. In order to utilize the space more effectively, complex IC chips comprise two or more concentric rings of pads, positioned in a stagger pattern. This staggering of the pads enables the conductive lines of the outer row of pads to fit between the pads in the inner row.
Please refer to FIG. 1. FIG. 1 is a diagram showing the structure of part of a conventional complex IC 100. As can be seen, the IC 100 comprises five pads 112, 114, 116, 122, 124, wherein pads 112, 114, 116 form the outer row of the IC 100 and pads 122, 124 form the inner row of the IC 100. Respectively coupled to the outer pads 112, 114, 116 are conductive lines 113, 115, 117, which fit between the inner pads, and respectively coupled to the inner pads 122, 124 are conductive lines 123, 125. As can be seen from the diagram, the width of the outer conductive lines 113, 115, 117 is restricted by the inner pads 122, 124.
In addition please refer to FIG. 2, which is a cross-section diagram of a pad structure 120 of the pad 122 shown in FIG. 2. As can be seen, the complete pad structure 120 comprises a plurality of layers, including a top metal Mtop 123, the pad 122, a via 20 for connecting the pad 122 and the top metal 123, and a metal layer Mtop−1 123a that contacts with the top metal 123 through vias 12 and 14. The via connection makes the top metal layer Mtop 123 have the same potential as Mtop−1 123a. As can be seen from the diagram, the two metal layers 123, 123a are both wider than the pad 122. Please note that either Mtop or Mtop−1 can be utilized for transmitting signals to circuitry of the IC chip 100.
The size (or width) of the conductive line (123 or 123a) affects the amount of power that the pad 122 can supply to the IC chip 100, with a wider conductive line supplying more power than a narrower conductive line. As can be seen from FIG. 1, if the pad pitch (distance between neighboring pads) is increased then the width of the conductive lines can be increased. There is therefore a trade-off between power and a number of pads of the IC chip 100. Furthermore, if the pad pitch decreases (pads are placed closer together), the problem of electro-migration is more likely to occur.